Unix Process Control
B.Ramamurthy
3/9/2003
B.Ramamurthy
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Process images in virtual
memory
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Creation of a process
Assign a unique pid to the new process.
Allocate space for all the elements of the
process image. How much?
The process control block is initialized. Inherit
info from parent.
The appropriate linkages are set: for
scheduling, state queues..
Create and initialize other data structures.
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Process Interruption
Two kinds of process interruptions: interrupt
and trap.
Interrupt: Caused by some event external
to and asynchronous to the currently running
process, such as completion of IO.
Trap : Error or exception condition generated
within the currently running process. Ex:
illegal access to a file, arithmetic exception.
Supervisor call: explicit interruption.
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Unix system V
All user processes in the system have as root
ancestor a process called init. When a new
interactive user logs onto the system, init
creates a user process, subsequently this user
process can create child processes and so on. init
is created at the boot-time.
Process states : User running , kernel running,
Ready in memory, sleeping in memory (blocked),
Ready swapped (ready-suspended), sleeping
swapped (blocked-suspended), created (new),
zombie , preempted (used in real-time
scheduling).
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UNIX SVR4 Process States
Similar to our 7 state model
2 running states: User and Kernel
„
transitions to other states (blocked, ready) must
come from kernel running
Sleeping states (in memory, or swapped)
correspond to our blocking states
A preempted state is distinguished from the
ready state (but they form 1 queue)
Preemption can occur only when a process is
about to move from kernel to user mode
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UNIX Process State Diagram
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Process and Context Switching
Clock interrupt: The OS determines if the
time slice of the currently running process is
over, then switches it to Ready state, and
dispatches another from Ready queue.
“Process switch”
Memory fault: (Page fault) A page fault
occurs when the requested program page is
not in the main memory. OS (page fault
handler) brings in the page requested,
resumes faulted process.
IO Interrupt : OS determines what IO action
occurred and takes appropriate action.
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Process and Context Switching
(contd.)
How many context switch occurs per process
switch?
Typically 1 Process switch : 100 context
switches
Process switch of more expensive than
context switch.
Read more on this.
This factor is very important for many system
design projects.
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Process and Context Switching
(contd.)
Process switch: A transition between two
memory-resident processes in a
multiprogramming environment.
Context switch: Changing context from a
executing program to an Interrupt Service
Routine (ISR). Part of the context that will be
modified by the ISR needs to be saved. This
required context is saved and restored by
hardware as specified by the ISR.
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Process and kernel context
process context
Application pgms
User mode
system calls
Kernel acts on behalf of user
kernel
kernel tasks
interrupt services
mode
kernel context
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U area
Process control block
Pointer to proc structure
Signal handlers related information
Memory management information
Open file descriptor
Vnodes of the current directory
CPU usage stats
Per process kernel stack
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Process Context
User address space,
Control information : u area (accessed
only by the running process) and
process table entry (or proc area,
accessed by the kernel)
Credentials : UID, GID etc.
Environment variables : inherited from
the parent
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UNIX Process Image
User-level context
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„
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Process Text (ie: code: read-only)
Process Data
User Stack (calls/returns in user mode)
Shared memory (for IPC)
Š only one physical copy exists but, with virtual
memory, it appears as it is in the process’s
address space
Register context
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UNIX Process Image
System-level context
„
Process table entry
Š the actual entry concerning this process in the
Process Table maintained by OS
„
„
Process state, UID, PID, priority, event awaiting, signals
sent, pointers to memory holding text, data...
U (user) area
Š additional process info needed by the kernel
when executing in the context of this process
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effective UID, timers, limit fields, files in use ...
Kernel stack (calls/returns in kernel mode)
Per Process Region Table (used by memory manager)
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Process control
Process creation in unix is by means of the system
call fork().
OS in response to a fork() call:
„ Allocate slot in the process table for new process.
„ Assigns unique pid.
„ Makes a copy of the process image, except for the
shared memory.
„ Move child process to Ready queue.
„ it returns pid of the child to the parent, and
a zero value to the child.
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Process control (contd.)
All the above are done in the kernel mode in the
process context. When the kernel completes these it
does one of the following as a part of the dispatcher:
„ Stay in the parent process. Control returns to the
user mode at the point of the fork call of the
parent.
„ Transfer control to the child process. The child
process begins executing at the same point in the
code as the parent, at the return from the fork
call.
„ Transfer control another process leaving both
parent and child in the Ready state.
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UNIX Process Creation
Every process, except process 0, is
created by the fork() system call
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„
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fork() allocates entry in process table and
assigns a unique PID to the child process
child gets a copy of process image of
parent: both child and parent are executing
the same code following fork()
but fork() returns the PID of the child to the
parent process and returns 0 to the child
process
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UNIX System Processes
Process 0 is created at boot time and
becomes the “swapper” after forking
process 1 (the INIT process)
When a user logs in: process 1 creates
a process for that user
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Process creation - Example
main () {
int pid;
cout << “ just one process so far”<<endl;
pid = fork();
if (pid == 0)
cout <<“im the child “<< endl;
else if (pid > 0)
cout <<“im the parent”<< endl;
else
cout << “fork failed”<< endl;}
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fork and exec
Child process may choose to execute some other
program than the parent by using exec call.
Exec overlays a new program on the existing
process.
Child will not return to the old program unless exec
fails. This is an important point to remember.
Why do we need to separate fork and exec? Why
can’t we have a single call that fork a new program?
See the enclosed sheets from Havilland and Salama’s
text.
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Example
if (( result = fork()) == 0 ) {
// child code
if (execv (“new program”,..) < 0)
perror (“execv failed “);
exit(1);
}
else if (result < 0 ) perror (“fork”); …}
/* parent code */
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